1. Field of the Invention
This invention pertains to resonant scan (flyback) deflection circuits and, more specifically, to means for protecting the output transistor in such a circuit from being damaged by excessive flyback voltage.
2. Description of the Prior Art
Horizontal deflection circuits for television receivers, picture monitors, computer terminal displays, etc., typically use a resonant scan (flyback) technique to create the horizontal deflection ramp signal. Such devices are designed to operate in various different formats in terms of scan rates, number of scanning lines, scan size, etc., depending on the particular display application or television standard for which they are designed. It is of course desirable to provide a single, versatile deflection circuit that can be used for any display format without the need to modify or recalibrate the circuit. However, such a wide range, variable rate resonant scan deflection circuit requires a B+ voltage source with an equally wide range. This subjects the horizontal output transistor to the risk of destruction from extremely large flyback voltage peaks produced, for example, during instrument turn-on or by free run transients. Other resonant scan deflection circuit failure modes, and prior art means for protecting the horizontal output transistor, are outlined in the background section of U.S. Pat. No. 4,241,296 to Barter, issued Dec. 23, 1980 and assigned to the assignee of the present invention.
The above-mentioned Barter patent discloses a horizontal deflection output circuit capable of operating at different scan rates, while at the same time protecting the horizontal output transistor from being damaged or destroyed by excessively high flyback voltage pulses. A pertinent portion of the Barter circuit is shown in FIG. 1. During flyback, the deflection yoke current causes a very high voltage (e.g., about 1200 V). to build up on the flyback capacitor connected across the output transistor's collector and emitter terminals. If the yoke current becomes too large before flyback occurs, the voltage across the output transistor (Q.sub.4) will exceed the device's maximum allowable V.sub.CE and destroy it. The output transistor is protected by a one-shot multivibrator comprised of transistor Q.sub.1, resistor R.sub.1 and capacitor C.sub.1. Capacitor C.sub.1 charges during the negative portion of the horizontal drive pulse applied to the input resistor R.sub.O. If the positive-going edge of the drive pulse does not occur before capacitor C.sub.1 charges to the voltage required to turn Q.sub.1 on, the voltage on capacitor C.sub.1 will turn on transistor Q.sub.1 and terminate the trace by turning off horizontal output transistor Q.sub.4 via Darlington transistors Q.sub.2 and Q.sub.3 and resistors R.sub.2 and R.sub.3. With increasing B+ voltage, capacitor C.sub.1 will charge faster and turn on Q.sub.1 sooner if the positive-going drive pulse edge does not occur in time.
The FIG. 1 drive circuit has relatively low gain, so it is designed to begin turning Q.sub.1 on relatively early, insuring that the output transistor will be switched off so that the flyback voltage will not be excessively high. However, this produces "jitter" in the deflection circuit, a condition in which the leading edge of the flyback pulse is time variant in a vibratory or random manner. In addition, the FIG. 1 circuit does not compensate for changes in the storage delay (i.e., turn-off delay time) of the deflection transistor at different scanning rates. The flyback voltage limit imposed by the circuit thus varies with the scanning rate, rather than remaining constant, and makes it necessary to establish on overly large "buffer zone" between the maximum permissible flyback voltage and the circuit-imposed limit.